In general, the price of grains is determined depending on the processability and the palatability of grains, and the price of grains of better quality is higher. For rice, for example, palatable rice of typically “Koshihikari”, “Hitomebore”, “Hinohikari” and “Akitakomachi” that are the best four varieties of rice harvested in 2000 in Japan is more accepted by consumers who need palatable rice, and the prices of these varieties of rice are high.
However, different rice is mixed into such high-quality rice by some dishonest traders, and the unfair mixed rice with a false indication is sold on the market. This is a matter of grave concern. Under the revised JAS Act that is in force from April 2001, rice traders have an obligation to express the variety of rice, the rice-producing district and the rice harvest year on all rice packages to be on the market. Given that situation, it is necessary to develop a technique of scientifically inspecting whether the indication given on rice packages fairly corresponds to the contents of the packages.
Heretofore, varieties of grains such as rice have been distinguished by the plant morphology, the grain morphology and the enzyme polymorphism in leaves and grains. At present, however, a variety of “Koshihikari” and its related varieties account for more than 70% of the overall rice yield in Japan, and this means that the rice production in Japan is mostly shared by such closely related varieties. In that situation, it is impossible to detect the presence or absence of any different variety mixed in a certain variety of rice according to the conventional method as above.
We, the present inventors have previously developed some techniques of rice variety discrimination through RAPD method or through a method of using STS primers, and have disclosed them in the Journal of the Food Science and Technology of Japan, Vol. 46, No. 3, pp. 117–122; Japanese Patent No. 3,048,149; and Japanese Patent Laid-Open No. 2001-95589.
These techniques make it possible to discriminate clearly a certain variety of rice from another variety that differs from it. However, when the discrimination band pattern of a different variety of rice to be discriminated from “Koshihikari” partly overlaps with that of “Koshihikari”, or when about 10 to 30% of a different variety of rice that gives a smaller number of discrimination bands than “Koshihikari” is mixed with “Koshihikari”, or when different several varieties of rice are mixed, it is difficult to individually detect and identify those different varieties of the mixed rice.
In checking a rice sample whether it contains mixed rice by the use of three or four types of primer sets, even when no DNA band specific to mixed rice has appeared in the band pattern of the sample, the probability that the sample is just the intended variety of high-quality rice only is, at present, at most 88 to 94% (=1−(0.5)3 to 4) and is not high enough.
In addition, when some bands specific to mixed rice have appeared in the DNA band pattern, the varieties of the mixed rice need to be identified in most cases. Therefore, the detection of only the discrimination bands that should not appear in the band pattern of the object variety of rice is insufficient for the information to identify the varieties of mixed rice, and further PCR is needed for individually identifying the varieties of mixed rice.
According to the conventional techniques heretofore known in the art, it is impossible to rapidly and accurately detect the presence or absence of mixed varieties in grains and to individually identify the mixed varieties in a simplified manner.